Electrolysis of fused salts



March 15, 1-938. 7 H. N. GILBERT 2,111,264

ELEGTROLYSIS 0F FUSED SALTS Original Filed Feb. 17, 1953 2 et 1INVENTOR. HARVEY N. GILBERT BY M Z 64M ATTORNEY March 15, 1938. N.GILBERT 2,111,264

ELECTROLYSIS OF FUSED SALTS Original Filed Feb. 17, 1933 2 Sheets-Sheet2 Fig.3

INVENTOR. HARVEY N. GILBERT BY 2 ATTORNEY small in order to avoid unduepower consumption Patented Mar. 15, 1938 UNITED STATES\ PATENT OFFICEELECTROLYSIS OF FUSED SALTS Harvey N. Gilbert, Niagara Falls, N. Y.,assignor to E. I. du Pont de Nemours & Company, Incorporated,Wilmington, DcL, a corporation of Delaware 10 Claims.

molten salts, it is necessary to have a relatively short distancebetween the electrodes. In the electrolysis of aqueous solutions, thedistance between the anode and cathode is of relatively small importancebecause of the relatively high conductivity of aqueous solutions.However, fused salts are relatively poorer conductors than aqueoussolutions of salts, and for this reason the distance between theelectrodes must be relatively per pound of metal obtained.

In the production of a metal by the electrolysis of a fused salt, it isusually necessary to use a mixture of a salt of the metal desired withone or more salt or salts of other metal or metals, in order for theelectrolyte to have a suitable melting point. When such a mixture ofsalts is electrolyzed, the various salts being present in suitableproportions, the desired metal will be produced in predominatingamounts. For example, a mixture containing approximately equal parts byweight of sodium chloride and calcium chloride electrolyzed in themolten state will produce metallic sodium with a small amount of calciumin solution. However, if the calcium chloride content is increased tobeyond about by weight, the amount of calcium produced will exceed thatwhich can be dissolved by the sodium with the result that calcium willbe formed as a separate phase. At the temperatures at which it isdesirable to operate this process, the calcium thus separated will be inthe solid state and hence will present difliculties where the cell isdesigned to remove the metal produced by flowing it out in the liquidstate. Furthermore, the solid calcium tends to build up on the cathodeand eventually will form a bridge across to the diaphragm and/ or anode,causing short circuits and lowering the cell eiiiciency.

When such a salt mixture is electrolyzed in a cell having a relativelyshort distance between the cathode and anode, especially where theelectrode surface is large, the preferential electrolysis of one metalcauses an impoverishment of the salt of that metal in the bath in thespace between the electrodes. Due to the relatively narrow space betweenthe electrodes, the natural difiusion and/or circulation of theelectrolyte is insufficient to bring fresh electrolyte into the spacefast enough to replace the salt of the metal preferentially removed bythe electrolysis. This changes the salt composition in the space betweenthe electrodes and causes excessive amounts of the undesired metal to bereleased. For example, in the above-mentioned production of sodium froma fused mixture of sodium chloride and calcium chloride, theimpoverishment of the electrolyte often causes formation of solidcalcium on the cathode with the undesirable results noted above.Furthermore, the change in the salt composition between the electrodeoften results in an increase in the melting point of the bath, whichmakes it necessary to operate the cell at a higher temperature. This isdisadvantageous because more electrical current is then required to keepthe cell temperature above the melting point of the electrolyte, and thecurrent efficicncy of the cell is correspondingly lowered.

An object of this invention is to provide a. means for preventingchanges in electrolyte composition between the electrodes of a fusedsalt electrolytic cell. A further object is to provide means foroperating fused salt electrolytic cells with a smaller space between theelectrodes, and

at lower temperatures than has heretofore been possible withoutdecreasing the efliciency of the cell. Other objects'will be hereinafterapparent.

These objects are attained in accordance with the present invention byproviding a positive means for circulating the fused salt electrolytethrough the space between the cathode and anode of the electrolyticcell, so as to increase the flow of electrolyte into or through saidspace to a rate sufiicient to prevent substantial change in compositionof electrolyte within said space.

Two methods of carrying out my invention are illustrated by the appendeddrawings. Figs. 1 and 3 are diagrammatic sectional views of fused saltelectrolytic cells. Fig. 2 is a perspective view of the cathode 3 of thecell illustrated by Fig. 1.

One method of carrying out my invention is illustrated by Figs. 1 and 2of the appended drawings. The fused salt electrolytic cell illustratedby Fig. 1 has a cylindrical brick-lined, steel casing 1. A cylindricalgraphite anode 2 projects upwardly through the bottom of the cell-casingl. The cathode 3 is a steel cylinder having two diametrically opposedsteel arms 4 which project outside the cell casing to serve as electrodeterminals. The cathode 3 is pierced with holes 5 at a large number ofpoints uniformly distributed over its surface, these holes 5 slantingupwardly towards the anode at an angle of about 45. A

perforated cylindrical steel diaphragm 6 is suspended about mid-way inthe annular space between the anode 2 and the cathode 3. The annularcollector ring 1 serves to support the diaphragm 6 and to collect moltenmetal which rises in the fused electrolyte from the cathode 3. Outlettube serves to carry the metal collected in collector ring I to theoutside of the cell. Gas dome 9 is for the purpose of carrying outgaseous anodic products formed by the electrolysis. The elements 6, I,8, and 9 are supported in the cell by means not shown.

Fig. 2 is a perspective view of the perforated cathode 3 of the cellillustrated in Fig. 1. The holes 5 are distributed uniformly over theperiphery of the cathode 3, and slant upwardly at an angle of about 45towards the axis of the cathode.

The cell illustrated by Fig. l is similar to that described in U. 8.Patent 1,501,756, issued to James C. Downs, with the exception that thecell illustrated by Fig. 1 has the cathode perforated as described,while in the cell described in the Downs patent the cathode has noperforations. In operating the ordinary Downs cell, with a cathode of agiven effective area, I have experienoed little or no trouble inimpoverishment of the electrolyte in the space between the electrodes,provided that the distance between the electrodes is not less than acertain minimum distance. I have discovered, however, that if the spacebetween the electrodes is reduced below the aforesaid minimum,impoverishment of the electrolyte in the space between the electrodeswill occur. a 1

I have discovered that the above mentioned disadvantages occurring infused salt electrolysis may be overcome by increasing the flow ofelectrolyte into or through the space between the electrodes, to a ratesufficient to prevent improverishment of electrolyte.

One method of increasing the electrolyte flow between the electrodes, inaccordance with my invention, is to provide the cell with a perforatedcathode as illustrated by Figs. 1 and 2 of the ap-' pended drawings. Ina cell thus equipped, the rise of the products of electrolysis in thespace between the electrodes, causes a steady flow of electrolytethrough the perforations 5 of the cathode 3 into the space between theelectrodes, at a rate suflicient to prevent impoverishment therein. Thenumber and size of holes in the perforated cathode will depend upon anumber of factors such as the composition of the electrolyte, rate offlow of the electrical current, the distance between the electrodes andthe effective area of the cathode. Although, for the purpose ofillustrating my invention, I have shown the perforations in the cathodesinclining upwardly at an angle, this is not essential; good results maybe secured if the holes 5 are at right angles to the surface of thecathode. However; it is preferable to incline the holes 5 upwardly,especially if the cathode is of substantial thickness.

In prior fused salt electrolytic cells, the relatively slow flow ofelectrolyte into the space between the electrodes was from the endstowards the middle of the space between the electrodes, and hence freshelectrolyte was brought in only at ends of the effective electrodesurfaces, while electrolysis occurred over the entire space. Theemployment of my perforated cathode insures that fresh electrolyte willbe continuously delivered to all parts of the space between theeffective surfaces of the electrodes.

Although it might appear obvious that the effective surface of thecathode would be reduced by making the perforations therein to suchextent that the cell efllciency would be lowered, I have found this notto be the case. On the contrary, it appears that possibly theperforations would increase the effective surface of the cathode.

Another method of carrying out my invention is illustrated by Fig. 3 ofthe appended drawings. Fig. 3 is adiagrammatic cross-section view of acell similar to that illustrated by Fig. 1 except that the cathode illin Fig. 3 has no perforations. A plurality of air-lift pumps, consistingof the elements II and I 2 are suspended by means not shown in the cell,at equidistant points around the cathode l0. Each of these pumpsconsists of .an air-inlet tube I2 inserted in a wider bent tube l I,open at both ends. The pumps are operated by passing a stream of air orother gas through the inlet tube l2. The passage of the gas upwardlyfrom tube l2 through the long arm of the bent tube ll causes a rapidflow of the molten electrolyte through tube ll. Since the lower end oftube i I is situated just below the space between the electrodes 2 andIII, the flow of the electrolyte through tube ll causes a rapid downwardflow of electrolyte in the space between the electrodes. This downwardflow is maintained at a rate suflicient to prevent impoverishment of theelectrolyte within the space between the electrodes, and yet is notpowerful enough to prevent the products of electrolysis from risingupwards into collector ring 1 and gas dome 9, respectively.

In operating fused salt electrolytic cells of the Downs type, for theproduction of sodium by electrolysis of a mixture of calcium chlorideand sodium chloride, I have experienced considerable difficulty withshort circuits caused by deposition of'calcium on the cathode andeventual bridging of the calcium deposits across to the diaphragm. Thesedifficulties are increased when it is attempted to narrow the spacebetween the electrodes. One effect of these short circuits was to causeserious fluctuations in the cell voltage, for instance, as shortcircuits occur, the voltage steadily drops. If measures are taken toremove the calcium deposits, for instance, by placing a new diaphragm inthe cell, the voltage is brought back to its original value, but in ashort time is again decreased by short circuits. These short circuitsoften cause the voltage to fluctuate as much as 0.6 volts. The shortcircuits not only cause voltage fluctuation, but also cause thediaphragm to corrode, making frequent diaphragm changes necessary.

I have found that by using a cell equipped with a perforated cathode asillustrated by Figs. 1 and 2 of the appended drawings, the cell voltageremains substantially constant, often not varying by more than about0.02 volt, diaphragm changes are required much less frequently, and theaverage cell production is increased by about 30 pounds of sodium perday.

Similar improvements may be obtained by using the air-lift pumps asillustrated by Fig. 3 of the appended drawings. In one case, a fusedmixture of calcium chloride and sodium chloride was electrolyzed in acell of the Downs type, to produce sodium and chlorine. During the first11 days of operation, the sodium production of this cell maintained ahigh average, although the voltage fluctuated to some extent, themaximum and minimum readings differing by about 0.32 volt. During thenext 14 days, considerable dimculty was experienced with short circuits.The voltage fluctuated by about 0.66 volt and the average voltage was0.44 volt less than during the preceding 11 days. In order to preventthe cell production from becoming unduly low, it was necessary to loosencalcium deposits and change diaphragms several times. On the 26th day ofoperation, a new diaphragm was placed in the cell and four air-liftpumps similar to those illustrated in Fig. 3 of the appended drawings,were installed and put into operation. The cell voltage immediately roseto the normal value and during the next 18 days, while the pumps wereoperated, the voltage did not vary by more than 0.06 volt. The averagedaily production also was increased by about 44 pounds per day, duringthe 18 days that the air-lift pumps were operated.

While I have illustrated my invention by showing two specific devicesfor causing a flow of the electrolyte into the space between theelectrodes in one specific type of fused salt electrolysis cell, myinvention is not restricted thereto. Other means for effecting said flowof electrolyte may be employed in the particular type of cell describedherein, without departing from the spirit and scope of my invention.Furthermore, similar means may be adapted for other types of fused saltelectrolytic cells as will be apparent to those skilled in designing andoperating such cells.

I prefer to produce-the increased circulation by means of a perforatedelectrode as illustrated by Figs. 1 and 2 of the appended drawings. Thisis a simple and effective manner in which to carry out my invention; itavoids the use of complicated apparatus, and requires no especialattention in operation. Furthermore, if this type of device forincreasing the cell circulation is used, it is not essential that thecathode be perforated but similar perforations or conduits may beprovided in the anode or in both the anode and the cathode. For example,in some types of fused salt electrolytic cells, it may be preferable ormore feasible to perforate the anode rather than the cathode. Ifdesired, a pumping device may be used to force electrolyte through suchperforations or conduits. Also, other means may be used to flow a streamor streams of the electrolyte into the space between the electrodes. Forexample, in place of the above described perforated cathode, a series ofrings, horizontally disposed one above the other, or a helical cathodemay be employed. Many other forms, equivalent to the perforated cathode,will be apparent; for ex ample, such may be found of woven wireconstruction, or may exist of a grating made of steel bars or rods.Also, a series of tubes leading into the space between the electrodesmay be provided, through which the electrolyte is caused to flow bymeans of one or more suitable pumping devices.

One advantage of my invention is that it makes possible the use of anarrower space between the electrodes of fused salt electrolytic cellsthan has been heretofore possible. The effect of narrowing the spacebetween the electrodes is to increase the yield of electrolysis productsper kilowatt hour of electrical current and thus to increase the overallefficiency of the cell. The positive circulation of electrolyte into thespace between the electrodes prevents impoverishment of any constituentof the electrolyte therein, and therefore insures uniformity of cathodicproducts. For example, in the electrolysis of a mixture of sodiumchloride and calcium chloride to produce sodium, my invention insuresuniform production of sodium containing a minimum amount of calcium.Furthermore, my invention makes possible the use of a higher proportionof calcium chloride in this electrolysis, while avoiding the formationof calcium in amounts greater than will dissolve in the sodium releasedat the cathode. Hence, it prevents the calcium from building up on thecathode and bridging across to the diaphragm and/or anodes to causeshort circuits.

A further advantage of my invention is that it enables the electrolysisof fused salt to be carried out at lower temperatures than has beenpossible heretofore. This is because my invention permits theelectrolysis to be carried out with a salt mixture nearer the eutecticpoint, without producing excessive amounts of undesired metal.

I claim:

1; A process comprising electrolyzing a mixture of fused salts in arelatively narrow, elongated zone of electrolysis, while fllowing theelectrolyte in a plurality of streams into said zone, the points ofentrance of said streams into said zone being substantially uniformlydistributed throughout at least a major portion of said zone.

2. A process comprising electrolyzing a mixture of fused salts in arelatively narrow, elongated zone of electrolysis, which zone issurrounded by a large body of electrolyte, while flowing the electrolyteinto said zone of electrolysis at a rate sufficiently high to maintainthe electrolyte composition throughout said zone substantially identicalwith the composition of the electrolyte which surrounds said zone andseparating a metallic electrolysis product from the electrolyte.

3. A process comprising electrolyzing a mixture of fused salts in arelatively narrow, elongated zone of electrolysis, which zone issurrounded by a large body of electrolyte, while flowing the electrolytefrom said large body in a plurality of streams substantially uniformlydistributed throughout at least a major portion of said zone.

4. A process comprising electrolyzing a fused mixture of calciumchloride and sodium chloride, in a relatively narrow, elongated zone ofelectrolysis, which zone is surrounded by a large body of theelectrolyte, while flowing the electrolyte in a plurality of streamsinto said zone of electrolysis at a rate suflicient to maintain thecomposition of the electrolyte within said zone substantially identicalwith the electrolyte com position surrounding said zone.

5. A process comprising electrolyzing a fused mixture of calciumchloride and sodium chloride, in a relatively narrow, elongated zone ofelectrolysis, which zone is surrounded by a large body of theelectrolyte, while flowing the electrolyte downwardly through said zoneof electrolysis at a rate sufficient to maintain the composition of theelectrolyte within said zone substantially identical with theelectrolyte composition surrounding said zone, while allowing theproducts of electrolysis to escape upwardly.

6. A process comprising electrolyzing a fused mixture of sodium chlorideand less than 71% by weight of calcium chloride, in a relatively narrow,elongated zone of electrolysis, which zone is surrounded by a large bodyof the electrolyte, while flowing the electrolyte in a plurality ofstreams into-said zone of electrolysis at a rate sufficient to maintainthe composition of the electrolyte withinsaid zone substantiallyidentical with the electrolyte composition surrounding said zone.

7. In a process for electrolyzing a mixture of fused salts, the methodcomprising flowing the electrolyte in a plurality of streams into thespace between the anode and cathode, said streams entering said space ata plurality of points substantially uniformly distributed over at leastone boundary of said space, which boundary is defined by an electrodeface.

8. In a process for electrolyzlng a mixture 6: ture of fused salts in arelatively narrow, eloni'used salts comprising an alkali metal halide,th gated zone of electrolysis, which zone is surmethod comprisingflowing the electrolyte in a" rounded by alarge body of electrolyte,while flowplurality of streams into the space between the t he,elecfiolyte downwardly through said zone anode and cathode, saidstreams entering said of electrolysis at a rate sufllcient to maintainthe space at a plurality of points substantially unielectrolytecomposition within said zone substanformly distributed over at least oneboundary of tially identical with the electrolyte composition saidspace, which boundary is defined by an elecsurrounding said zone, whileallowing the prodtrode face. ucts oi electrolysis to escape upwardly.

9. In a process for electrolyzing a mixture of HARVEY N. GILBERT.

fused salts comprising an alkali metal halide, the method comprisingflowing the electrolyte in a plurality of streams upwardly, into thespace between the anode and cathode, said streams entering said space ata plurality of points substantially uniformly distributed over an areawhich is substantially parallel to the cathode.

10. A process comprising electrolyzing a mix-,

